Surface Segregation of Methyl Side Branches Monitored by Sum Frequency Generation (SFG) Vibrational Spectroscopy for a Series of Random Poly(ethylene-co-propylene) Copolymers

Abstract

The surface composition and surface chain conformation of a series of random aspecific poly(ethylene-copropylene) rubber copolymers (aEPR) was quantified by sum frequency generation surface vibrational spectroscopy (SFG). All of the copolymers are found to preferentially orient side-branch methyl groups out of the surface. As the ethylene content of the copolymer increases, the number of methyl groups contributing to the sum frequency signal decreases. However, the percentage of methyl groups oriented out of the surface, relative to the bulk concentration of methyl groups, increases. This surface excess of oriented methyl groups is proposed to be a result of decreased steric hindrances between adjacent methyl groups in ethylene-rich copolymers. Additionally, analysis of the CH 2 bands in the SFG spectra suggests that the CH 2 units at the surface become more oriented toward the surface normal and adopt a trans configuration as the ethylene content increases.

Combined Atomic Force Microscopy and Sum Frequency Generation Vibrational Spectroscopy Studies of Polyolefins and Hydrogels at Interfaces

“…Analysis of the SFG spectra indicated that the most likely scenario is (1) and that ethylene-rich aEPR copolymers orient excess methyl branches at the polymer/air interface compared to aPP. 7 This result from aPP and aEPR can be put into perspective with SFG results obtained from other relatively simple polymers -polystyrene 15,16,17,18 , poly(hydroxyethyl)methacrylate, 9 and polymethylmethacrylate 19 where it has been observed that bulky low surface energy side branches tend to order at the polymer/air interface. In comparing these results to results from liquid interfaces, one must take into account a major difference between polymers and molecules in liquids -that the configuration of a polymer chain is restricted by the connectivity of chain segments along the backbone.…”

“…In each of these spectra, the strongest resonant feature is at 2883cm" 1 and is assigned as the CH 3 (s) stretch from the methyl side branches. 13,7 The feature at 2968cm' 1 is assigned to the CH 3 (a) stretch from the side branch. The features at 2850cm' 1 and 2920cm' 1 are assigned as the CH 2 (s) and CH 2 (a) stretches, respectively, from the polymer backbone.…”

“…In recent years, AFM has been proven as a versatile technique that can be used to map the surface morphology/topography and to probe the the visible beam (dty, 532 nm) and the tunable infrared beam (flfc, 2000 cm 1 to 4000 cm" 1 ) has been described elsewhere. 7 The interface specificity of SFG spectroscopy is a result of selection rules obtained under the electric-dipole approximation. The sum frequency signal, I(ω 5 ), is proportional to the square of the nonlinear susceptibility of the material being measured, %b\ a 27 component tensor (Eq.…”

Atomistic Simulation of Orientation of Methyl Groups and Methylene Bisectors, and Surface Segregation, in Freely Standing Thin Films of Atactic Poly(ethylene-co-propylene)

“…In Table 4a,b, we determine various ratios obtained from the simulation results that will be helpful in comparison with the experimental work. 5 In each table, a different surface region is identified. In Table 4a, the gaslike side of the Gibbs dividing plane is defined as the surface ∼10 Å thick.…”

Solvent Vapor Induced Ordering and Disordering of Phenyl Side Branches at the Air/Polystyrene Interface Studied by SFG

“…6 At the air interface, a number of SFG studies have shown that hydrophobic side branches are wellordered and are oriented away from the polymer bulk. [6][7][8][9][10][11] Two factors seem to be responsible for this observation: (1) air is considered hydrophobic, and thus hydrophobic side branches should be favored; and (2) there is a depletion in density (an increase in free volume) in the top few angstroms of the surface region, 12 which favors a large concentration of bulky side branches. At the polymer/liquid interface, the change in density between the polymer phase and the liquid phase is not as great as at the polymer/air interface, so side branch ordering based on low density may not play as large a role.…”